Some patients who have heart failure, and some of those at risk for developing it, receive interventions intended to temporarily assist the heart before or during a medical or surgical procedure and/or during a recovery period. The intervention typically lasts for less than a week, but can continue for several weeks. These interventions include pharmaceuticals and/or medical devices, including cardiac-assist devices.
Some cardiac-assist devices include a pump to supplement the heart's pumping action. By assuming some of the heart's pumping function, these “blood pumps” unload the heart, helping it to recover. Cardiac-assist devices can be temporary or permanent.
Some blood pumps have an extracorporeal (i.e., outside the body) impeller to drive blood flow. Some of these extracorporeal blood pumps connect to a patient's heart and blood vessels directly through the exposed chest using relatively large-diameter tubes (cannulas). Such procedures, performed by cardiac surgeons, are invasive and may require cardiopulmonary bypass. They are, unfortunately, associated with significant complications. Some other extracorporeal blood pumps connect to the patient using relatively wide catheters or cannulas, inserted through peripheral blood vessels.
Some other blood pumps are percutaneous, wherein the impeller (and in some devices, the pump's motor) temporarily reside within the patient. These blood pumps are often coupled to a catheter, and are consequently referred to as “catheter blood pumps.” Some catheter blood pumps are inserted into the patient using established cath-lab techniques, wherein they are advanced through the vascular system (typically the femoral artery) to a patient's heart. This approach is significantly less invasive than cardiac surgery or other relatively complicated procedures.
It is desirable for a catheter blood pump to have as small a diameter as possible, preferably less than 16 Fr, and more preferably less than 12 Fr, to minimize trauma to the vasculature or trauma associated with the surgery performed for insertion. It is also desirable for such a pump to have a large pumping capacity, preferably 2 liters per minute or even more, to provide sufficient circulation for a patient. Additionally, such a pump must avoid, to the extent possible, damaging the blood in the form of hemolysis (i.e., destruction of red blood cells).
Indeed, notwithstanding its attractiveness as a less-invasive alternative, most designs for percutaneously-inserted blood pumps exhibit one of more of the following shortcomings:                a limited pump flow;        some degree of hemolysis;        require the use of a large catheter/cannular, with a risk of ischemia; and        they are expensive.        
There have been previous attempts, mostly unsuccessful, to increase the flow rate through catheter blood pumps. Simply increasing the rotation speed of the pump's impeller will increase the flow rate. However, the increased speed results in additional power requirements, which in turn may increase the size and electrical demands of the motor. In devices that use a flexible drive cable to drive the pump's impeller (rather than an in-vivo motor sited near the impeller), the increased motor speed may require an increase in the size and stiffness of the flexible drive cable. Furthermore, the increased speed of the impeller causes increased shear stress on the blood, resulting in increased hemolysis. Also, greater motor speeds increase the likelihood of drive-cable failure.
As mentioned above, catheter blood pumps are usually advanced to the heart through the vascular system. Consequently, there is a limit as to the acceptable diameter of the largest feature of the catheter blood pump. Consider that such a blood pump typically includes various tubes, an impeller housing, an impeller, and a drive cable. Since the impeller is rotating at high speed (thousands of rpm), it is important that the impeller does not come into contact with the patient's anatomy or other parts of the blood pump (e.g., tubing, impeller housing, etc.). For a pump having a fixed-diameter, non-foldable/non-expandable impeller, an outermost tube, typically called a sheath is typically the largest-diameter feature, whereas other elements of the blood pump (e.g., impeller housing, impeller, etc.) that are intended to be introduced into the vasculature are contained within the sheath. As a consequence, the diameter of the impeller is necessarily smaller than the sheath and smaller than the impeller housing. This typically results in an impeller having a diameter in the range of 9 to 12 Fr, which presents a significant limitation to generating pump flows greater than about 2 liters/minute.
In recognition of this problem, catheter blood pumps having an expandable impeller have been proposed. An expandable impeller collapses to a very small diameter (e.g., by folding/bending/hinging the impeller blades, etc.) to fit within a tube (e.g., sheath, etc.) for insertion into the body, and delivery via the vasculature to the aorta or heart. Once the pump is positioned, the expandable impeller is freed from the confines of the tubing (e.g., such as by partially retracting the tubing, etc.) and then expands to a larger diameter that could not otherwise be accommodated by the patient's vasculature. Usually, the expandable impeller is accompanied by an expandable impeller housing that is freed from the sheath/tubing at the same time as the expandable impeller, enabling both to expand in diameter.
This larger-diameter impeller develops, at least theoretically, a notably greater pumping capacity than would otherwise possible. However, expandable-impeller designs present significant implementation challenges, including the design of the impeller itself, as well as issues related to repeatedly and accurately controlling the gap between the rapidly rotating impeller blades and the surrounding impeller housing. Furthermore, experimentation and simulation (i.e., computational fluid dynamics) have shown that most proposed expandable impeller designs are relatively inefficient for generating blood flow as compared to what is achievable with some fixed-diameter impeller designs.